From the EP 0,787,875 A2 there is known a keyless securing (or security) system for a locking system of a motor vehicle, that comprises an interrogation unit which is allocated to the motor vehicle and which acts on the locking system, and several mobile response units that are to be carried by and on users, whereby the interrogation unit transmits an interrogation signal and tests whether at least one of the response units responds to the interrogation signal with a response signal that contains, as information, a code that is authorized for deactivating or unlocking the locking system. If this is the case, the locking system is deactivated or unlocked by the security system.
The essential disadvantage of this security system is seen in that unauthorized users can gain access to the vehicle via a relay intervention without being noticed by the rightful user, in that they establish a radio link via relay stations between the interrogation unit and the response unit of the authorized user, and deliver the code that is authorized for the unlocking or deactivating to the interrogation unit via this radio link.